It is known from the technology of chemical process engineering to connect several on/off valves in parallel for controlling a quantity flow. These valves are either opened or closed depending on the respective nominal amount. In this case, the cross sections of dosing can be graded, for example, according to a binary series 2.sup.n. With the use of an increasing number n of the valves, the resolution capability is improved. This binary dosing method has certain advantages, that is, the relatively simple design of the valve and the fact that actuation of the valves is not as frequently necessary. Further, there is the possibility that, when one or several valves fail, the redundancy of the remaining valves may be utilized. The method, however, has the disadvantage that, for many purposes, for example, in controlling gas turbines, a very large number of valves is required in order to obtain a sufficiently sensitive dosing. In practice, this large number cannot be realized.
The German Pat. No. 1,919,489 describes a device for dosing fuel for gas turbines, wherein a dosing valve is provided which assumes only two positions, namely, a position for the minimum and a position for the maximum required fuel amount. When the switching frequency remains constant, the time for the position assigned to the maximum required fuel amount is changed according to the desired fuel amount. Moreover, variable stops for the valve tappet are provided in order to be able to adjust a continuous base amount in addition to the pulse width- or duration-modulated amount.
In order to establish the absolute minimum amount, an adjustable throttle is provided parallel to the dosing valve. The absolute maximum amount is limited by means of another throttle which is also adjustable and is connected in series with the pulse duration-modulated valve. The linear allocation between the effective cross section of dosing and the flow is obtained by keeping constant the pressure difference above the cross section of dosing. In some types of applications, such as in the control of gas turbines, it may be necessary to automatically adjust the base amount, and thus the stops, in order to widen the dosing range. Such a possibility does not exist in this known proposal; however, the positioning device to be additionally provided for this purpose as an obvious measure would form a complex electromechanical system which would essentially cancel the advantage of the pulse duration-modulated dosing system, which is very simple by itself. Moreover, due to an insufficient redundancy, the safety factor in this dosing system is extremely low when the valve fails.